Avoid invitation to "unmark as proofread" since it is not allowed

CSS has been adjusted to avoid to convey the idea that proofread marks
can be removed. Once the user marks a message as proofread, the
mark will not be highlighted on hover and the cursor will not turn into
a hand.

Change-Id: Ifec127376fb4d8f69b575265c11fd7b1e06e83d8
Merge "Avoid invitation to "unmark as proofread" since it is not allowed"